One of the most fundamental chemical systems in living cells is the one by which hereditary information is translated into proteins, substances which determine a cell's properties and functions. A key part of the specificity of protein synthesis resides in the initial steps of the process, the activation and transfer of an amino acid to its respective tRNA. A great deal remains to be learned about the enzymatic mechanism of amino acid activation, about the in vivo properties of the system which catalyzes these reactions, and about control of the process. The objectives of this project are 1) to study the properties and mechanism of a highly purified mammalian enzyme which catalyzes the process of amino acid activation; 2) to determine the structure and functional significance of an aminoacyl-tRNA synthetase complex isolated from liver which contains all the enzymes involved in this process; 3) to study the enzymes involved in biosynthesis and turnover of the terminal trinucleotide sequence which is required for the biological properties of transfer RNA; 4) to investigate the control of amino acid activation which may be exercised at the level of the biosynthesis of transfer RNA or at the level of attachment of the amino acid to the tRNA.